In recording on photoinic materials, such as silver halide films, printing plates, photoresists, etc. it is known that the rate of exposure, or dwell time of the recording spot (typically a laser spot), is of little importance as long as the total exposure is correct. This is the well-known "Law of Reciprocity". The exposure is defined as the product of the power of the light multiplied by the time. The power is normally measured in Watts and exposure is Joules or Watt-Seconds. When recording on materials known as thermal, or heat-mode materials, the rate at which the exposure is delivered is crucial, since a low exposure rate (low power for a long time) will not cause the desired increase in temperature, as most the heat will dissipate. On the other hand shortening the exposure time and using a very high power can cause the exposed material to break down or ablate, creating debris and not functioning properly (unless the material is designed to operate by ablation). This problem does not exist in photonic materials as they usually require significantly lower power. If any exposure system has to have a specified scanning rate (to achieve a desired throughput or productivity) and the material has a specified sensitivity (usually specified in Joules/cm.sup.2), these two parameters uniquely set the exposure power, since if during one second X cm.sup.2 need to be exposed, the power has to be "X" times the material sensitivity. For most materials used in thermal imaging the sensitivity is in the range of 0.1-1 joule/cm.sup.2 and writing rates are 10-100 cm.sup.2 /sec. This determines the writing power to be in the range of 1-100 W. If this power level is delivered in a single laser beam for writing high resolution features (1-20 microns) the power density (defined as Watts/cm.sup.2) is very high and causes ablation. Prior art solutions involve splitting the laser beam into many parallel beams (or using many parallel lasers) in order to reduce the power density per spot. Another solution, shown in FIG. 1, is to use spots which are larger than the required addressability, shown as "a" in FIG. 1. Digital images are made up of pixels and normally addressability is a single pixel. The disadvantage of the latter method is loss of resolution as the spot is larger than a single pixel. If the first method is used it is difficult to change the power density once the power was set (to achieve a desired imaging speed). Another method is to pulse the lasers in order to increase power density, however it lowers the reliability of the lasers. For devices required to image a wide range of thermal materials it is desired to be able to vary the power density without affecting resolution, power or writing speed. It is also sometimes desired to achieve high power densities without change in resolution, power or laser duty cycle, in order to use ablative recording materials. The ideal exposure method will allow the power density to be changed from very high (for ablative materials, typically requiring 1 MV/cm.sup.2) to low (for chemical reactions, typically requiring under 200 KW/cm.sup.2)